Method of detecting a substrate in a carrier head

ABSTRACT

A carrier head has a base, a flexible membrane, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that contacts an upper surface of the flexible membrane so that if a substrate is attached to the lower surface of the flexible membrane when the first chamber is evacuated, the valve is actuated to generate a signal to the substrate detection system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. application Ser. No. 09/441,928, filed on Nov. 17, 1999, issued asU.S. Pat. No. 6,663,466, the entire disclosure of which is incorporatedherein by reference.

BACKGROUND

The present invention relates generally to chemical mechanical polishingof substrates, and more particularly to the detection of a substrate ina carrier head.

Integrated circuits are typically formed on substrates, particularlysilicon wafers, by the sequential deposition of conductive,semiconductive or insulative layers. After each layer is deposited, thelayer is etched to create circuitry features. As a series of layers aresequentially deposited and etched, the outer or uppermost surface of thesubstrate, i.e., the exposed surface of the substrate, becomesincreasingly non-planar.

Chemical mechanical polishing (CMP) is one accepted method ofplanarizing a substrate surface. This planarization method typicallyrequires that the substrate be mounted to a carrier or polishing head.The exposed surface of the substrate is then placed against a rotatingpolishing pad or moving polishing belt. The polishing pad may be a“standard” pad with a durable roughened surface, or a fixed-abrasive padwith abrasive particles embedded in a binder. The carrier provides acontrollable load on the substrate to press it against the polishingpad. In addition, the carrier may rotate to affect the relative velocitydistribution over the surface of the substrate. A polishing slurry,including at least one chemically-reactive agent, and an abrasive if astandard pad is being used, may be distributed over the polishing pad.

Typically, the carrier head is used to remove the substrate from thepolishing pad after the polishing process has been completed. Thesubstrate is vacuum-chucked to the underside of the carrier head. Whenthe carrier head is retracted, the substrate is lifted off the polishingpad.

One problem that has been encountered in CMP is that the substrate maynot be lifted by the carrier head. For example, if the surface tensionbinding the substrate to the polishing pad is greater than the forcebinding the substrate to the carrier head, then the substrate willremain on the polishing pad when the carrier head retracts. Also, if adefective substrate fractures during polishing, then the carrier headmay be unable to remove the fractured substrate from the polishing pad.

A related problem is that the attachment of the substrate to the carrierhead may fail, and the substrate may detach from the carrier head. Thismay occur if, for example, the substrate was attached to the carrierhead by surface tension alone, rather than in combination withvacuum-chucking.

As such, an operator may not know that the carrier head no longercarries the substrate. The CMP apparatus will continue to operate eventhough the substrate is no longer present in the carrier head. This maydecrease throughput. In addition, a loose substrate, i.e., one notattached to a carrier head, may be knocked about by the movingcomponents of the CMP apparatus, potentially damaging the substrate orthe polishing pad, or leaving debris which may damage other substrates.

Another problem encountered in CMP is the difficulty of determiningwhether the substrate is present in the carrier head. Because thesubstrate is located beneath the carrier head, it is difficult todetermine by visual inspection whether the substrate is present in andproperly attached to the carrier head. In addition, optical detectiontechniques are impeded by the presence of slurry.

A carrier head may include a rigid base having a bottom surface whichserves as a substrate receiving surface. Multiple channels extendthrough the base to the substrate receiving surface. A pump or vacuumsource can apply a vacuum to the channels. When air is pumped out of thechannels, the substrate will be vacuum-chucked to the bottom surface ofthe base. A pressure sensor may be connected to a pressure line betweenthe vacuum source and the channels in the carrier head. If the substratewas not successfully vacuum-chucked to the carrier head, then thechannels will be open and air or other fluid will leak into thechannels. On the other hand, if the substrate was successfullyvacuum-chucked to the carrier head, then the channels will be sealed andair will not leak into the channels. Consequently, the pressure sensorwill measure a higher vacuum or lower pressure when the substrate issuccessfully vacuum-chucked to the underside of the carrier head ascompared to when the substrate is not attached to the carrier head.

Unfortunately, there are several problems with this method of detectingthe presence of a substrate in the carrier head. Corrosive slurry may besuctioned into the channels and contaminate the carrier head. Inaddition, the threshold pressure for determining whether the substratehas been lifted from the polishing pad must be determinedexperimentally.

Accordingly, it would be useful to provide a CMP system capable ofreliably sensing the presence of a substrate in a carrier head. It wouldalso be useful if such a system could operate without exposing theinterior of the carrier head to contamination by a slurry.

SUMMARY

In one aspect, the invention is directed to a carrier head that has abase, a flexible member that defines a first chamber and has a lowerface that provides a substrate receiving surface, and a valve in thecarrier head that forms part of a substrate detection system. The valveincludes a valve stem that contacts an upper surface of the flexiblemembrane so that if a substrate is attached to the lower surface of theflexible membrane when the first chamber is evacuated, the valve isactuated to generate a signal to the substrate detection system.

Implementations of the invention may include the following features. Thevalve may be positioned in a passage that fluidly couples the firstchamber to a second chamber. The valve may be biased in an open orclosed position, and actuation of the valve may close or open the valve.The valve stem may extend through an aperture in a support structure,and may project slightly beyond a lower surface of the supportstructure. The support structure may be movable relative to the base.The valve may be biased by a spring, and the spring constant of thespring may be selected so that the force from the spring is sufficientto counteract a force from a flexible membrane when the substrate is notattached, but is insufficient to counteract a force from a flexiblemembrane when the substrate is attached. The valve stem may contacts theupper surface of the flexible membrane if the first chamber isevacuated. The flexible membrane may wrap around a lower portion of thevalve if the substrate is not present.

In another implementation, the carrier head has a base, a flexiblemember that defines a first chamber and has a lower face that provides asubstrate receiving surface, and a valve in the carrier head that formspart of a substrate detection system. The valve includes a valve stemthat projects past a support surface, so that if the first chamber isevacuated and a substrate is attached to the lower surface of theflexible membrane, the substrate abuts the support surface and actuatesthe valve.

In another implementation, the carrier head has a base, a flexiblemember that defines a first chamber and has a lower face that provides asubstrate receiving surface, and a plurality of valves in the carrierhead that form part of a wafer detection system. If a substrate isattached to the flexible membrane when the first chamber is evacuated,either of the valves may be actuated to generate a signal to the waferdetection system.

In another implementation, the carrier head has a base, a flexiblemember that defines a first chamber and has a lower face that provides asubstrate receiving surface, and a plurality of valves in the carrierhead that form part of a wafer detection system. If a substrate isattached to the flexible membrane when the first chamber is evacuated,both of the valves must be actuated to generate a signal to the waferdetection system.

In another implementation, the carrier head has a base, a flexiblemember that defines a first chamber and has a lower face that provides asubstrate receiving surface, a second chamber, a passage through thebase between the first and second chambers, a first valve that is biasedopen and actuates to close the passage if the first chamber is evacuateda substrate is attached to the flexible membrane when the first chamber,and a second valve connected in series with the first valve, the secondvalve biased closed and actuatable to open the passage if the secondchamber is evacuated.

Advantages of the invention include the following. The CMP apparatusincludes a sensor to detect whether the substrate is properly attachedto the carrier head. The sensor is less prone to false alarms.

Other advantages and features of the invention become apparent from thefollowing description, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a chemical mechanicalpolishing apparatus.

FIG. 2 is partially a schematic cross-sectional view of a carrier headwith a flexible membrane and a chamber, and partially a schematicdiagram of a pneumatic control system for the carrier head.

FIG. 3A is an expanded view of the valve from the carrier head of FIG.2.

FIG. 3B is a view of the carrier head of FIG. 3A with an attachedsubstrate.

FIG. 4 is a graph showing pressure as a function of time in a CMPapparatus using the carrier head of FIG. 2.

FIG. 5 is a schematic cross-sectional view of a carrier head thatincludes multiple valves connected in parallel.

FIG. 6 is a schematic cross-sectional view of a carrier head thatincludes multiple valves connected in series.

FIG. 7 is a schematic cross-sectional view of a carrier head in whichvalves are separated by a diaphragm.

FIG. 8 is a schematic cross-sectional view of a carrier head in whichvalves are biased in opposite directions.

Like reference numbers are intended in the various drawings to indicatelike elements, although some elements in different implementations mayhave different structures, operations or functions.

DETAILED DESCRIPTION

Referring to FIG. 1, one or more substrates 10 will be polished by achemical mechanical polishing (CMP) apparatus 20. A complete descriptionof a CMP apparatus can be found in pending U.S. Pat. No. 5,738,574, theentire disclosure of which is hereby incorporated by reference. The CMPapparatus 20 includes a series of polishing stations 25 and a transferstation 27.

Each polishing station 25 includes a rotatable platen 30 on which isplaced a polishing pad 32. Each polishing station may further include anassociated pad conditioner apparatus 34 to periodically recondition thepolishing pad surface. Each polishing station can also include acombined slurry/rinse arm 36 to supply a slurry 38 containing an activeagent (e.g., deionized water for oxide polishing), abrasive particles(e.g., silicon dioxide for oxide polishing) and a chemically-reactivecatalyzer (e.g., potassium hydroxide for oxide polishing) to the surfaceof polishing pad 32.

The CMP apparatus 20 also includes a rotatable multi-head carousel 40that supports four carrier heads 100. Three of the carrier heads receiveand hold substrates and polish them by pressing them against thepolishing pad 32 on platen 30 of polishing stations 25. One of thecarrier heads receives a substrate from and delivers the substrate totransfer station 27. The carousel can rotate to orbit the carrier heads,and the substrates attached thereto, between the polishing stations andthe transfer station. Each carrier can be independently rotated aboutits own axis, and independently laterally oscillated by a drive shaft42.

Generally, carrier head 100 holds the substrate against the polishingpad and evenly distributes a force across the back surface of thesubstrate. The carrier head also transfers torque from the drive shaftto the substrate and ensures that the substrate does not slip frombeneath the carrier head during polishing.

Referring to FIG. 2, carrier head 100 includes a housing hub 102, a base104, a loading chamber 108, a retaining ring 110, and a substratebacking assembly 112. Descriptions of similar carrier heads may be foundin U.S. Pat. No. 5,957,751, and in U.S. Pat. No. 6,277,014, each ofwhich is incorporated herein by reference in its entirety.

The housing hub 102 is connected to drive shaft 42 to rotate therewithabout an axis of rotation which is substantially perpendicular to thesurface of the polishing pad. Three passages 130, 132 and 134 are formedthrough housing hub 104 for pneumatic control of the carrier head.

Base 104 includes a gimbal mechanism 106 and an outer clamp ring 144.The vertical position of base 104 relative to housing hub 102 iscontrolled by loading chamber 108. Chamber 108 also controls thedownward pressure on base 104 and retaining ring 110. Loading chamber108 is sealed by a diaphragm 140 that is clamped to housing hub 102 byan inner clamp ring 142 and clamped to base 104 between outer clamp ring144 and flexure ring 152. Outer clamp ring 144 includes an inwardlyprojecting flange 146 which extends over a lip of housing hub 102 toprevent over-extension of the carrier head and to prevent slurry fromcontaminating diaphragm 140.

A first pump or pressure source 52 a may be connected to loading chamber108 via passage 130 in housing hub 102. If pump 52 a pumps fluid intoloading chamber 108, the volume of the chamber will increase and base104 will be pushed downwardly. On the other hand, if pump 52 a pumpsfluid out of loading chamber 108, the volume of chamber 108 willdecrease and base 104 will be pulled upwardly.

Gimbal mechanism 106 permits base 104 to move with respect to housinghub 102 so that the retaining ring may remain substantially parallelwith the surface of the polishing pad. Gimbal mechanism 106 includes agimbal rod 150 and a flexure ring 152. Gimbal rod 150 may slidevertically in passage 132 in housing 102 so that base 104 can movevertically with respect to housing 102. However, gimbal rod 150 preventsany lateral motion of base 104 with respect to housing 102. A firstpassage 154 can be formed through gimbal rod 150, and a second passage156 can be formed through gimbal rod 150, flexure ring 152 and outerclamp ring 144 for pneumatic control of the carrier head.

Retaining ring 110 may be secured at the outer edge of base 104.Retaining ring 110 can have a flat bottom surface 126, or the bottomsurface can include channels to permit slurry flow. When fluid is pumpedinto chamber 108 and base 104 is pushed downwardly, retaining ring 110is also pushed downwardly to apply a load to polishing pad 32. An innersurface of 124 retaining ring 110 restrains the substrate from lateralmotion.

A membrane 162 may be clamped to a lower surface of base 104 by a clampring 164 to form an annular bladder 160. A passage 166 extends throughclamp ring 164 and is aligned with passage 156 in base 104. A secondpump or pressure source 52 b can connected to bladder 160 via passage134 in housing hub 102, passage 156 in base 104, and passage 166 inclamp ring 164. If pump 52 b forces a fluid into bladder 160, thenbladder 160 will expand downwardly. On the other hand, if pump 52 bevacuates fluid, then bladder 160 will contract. As discussed below,bladder 160 may be used to apply a downward pressure to supportstructure 114 and flexible membrane 118.

The substrate backing assembly 112 includes a flexible membrane 118, asupport ring 116, a support structure 114, and a spacer ring 128. Eachof these elements will be explained in greater detail below.

Flexible membrane 118 is a generally circular sheet formed of a flexibleand elastic material with a central portion 170 and a peripheral portion172 that extends between spacer ring 128 and support plate 114. Thecentral portion 170 of flexible membrane 118 extends below supportstructure 114 to provide a mounting surface for the substrate. An inneredge of the peripheral portion 172 is folded back over the perimeter ofthe central portion 170 to form an expandable lip 174, as discussed inU.S. Pat. No. 6,210,255, the entirety of which is incorporated herein byreference. An outer edge of membrane 118 is clamped between retainingring 110 and outer clamp ring 144 to define a pressurizable chamber 120.

A third pump or pressure source 52 c can be connected to chamber 120 viapassage 154 in gimbal rod 150. If pump 52 c forces a fluid into chamber120, then the volume of the chamber will increase and flexible membrane118 will be forced downwardly. On the other hand, if pump 52 c evacuatesair from chamber 120, then the volume of the chamber will decrease andthe membrane will be drawn upwardly.

Spacer ring 128 is an annular body positioned between support structure114 and retaining ring 110 to maintain the proper shape of flexiblemembrane 118. Spacer ring 128 can rest on the lip portion of flexiblemembrane 118.

Support ring 116 is an annular piece with a C-shaped cross-section thatrests inside chamber 120 on flexible membrane 118. The central portion170 of flexible membrane 118 can include an inwardly extending flap 176that engages support ring 116 to maintain the proper shape of flexiblemembrane 118.

Support structure 114 also rests inside chamber 120 on flexible membrane118. The support structure 114 includes a disk-shaped plate portion 180with a plurality of unillustrated apertures, an outwardly extendingflange portion 182 that extends over support ring 116, and a downwardlyextending flange portion 184 that extends between support ring 116 andperipheral portion 172 of flexible membrane to rest on the centralportion 170 of the flexible membrane.

The CMP apparatus of the present invention is capable of detectingwhether a substrate is properly attached to carrier head 100. If the CMPapparatus detects that the substrate is missing or is improperlyattached to the carrier head, the operator may be alerted and polishingoperations may be automatically halted.

Three pressure sensors or gauges 56 a, 56 b and 56 c may be connected tothe fluid lines between pumps 52 a, 52 b and 52 c, and chambers 108,160, and 120, respectively. Controllable valves 58 a, 58 b and 58 c maybe connected across the fluid lines between pressure gauges 56 a, 56 band 56 c, and pumps 52 a, 52 b and 52 c, respectively. Pumps 52 a-52 c,pressure gauges 56 a-56 c and valves 58 a-58 c may be appropriatelyconnected to a general-purpose digital computer 60. Computer 60 mayoperate pumps 52 a-52 c, as described above, to pneumatically powercarrier head 100 and to vacuum-chuck a substrate to the bottom of thecarrier head. In addition, computer 60 may operate valves 58 a-58 c andmonitor pressure gauges 56 a-56 c, as described in more detail below, tosense the presence of the substrate in the carrier head.

Referring to FIGS. 3A and 3B, the carrier head 100 includes amechanically actuated valve 200 to provide the carrier head with a waferdetection capability. In one implementation, passage 156 is connected toa chamber 220 in flexure ring 152, and valve 200 is positioned near thecenter of the carrier and extends between chamber 220 and chamber 120.In this implementation, valve 200 includes a valve stem 202, an annularflange 204 that extends radially outwardly from the valve stem 202, anO-ring 206, and a spring 214. Valve stem 202 extends through an aperture208 in flexure ring 152 between valve chamber 220 and lower chamber 120,with valve flange 204 positioned in valve chamber 220. The portion ofvalve stem 202 that extends into lower chamber 120 passes through anaperture 210 in support structure 114. When lower chamber 120 isevacuated and support structure 114 is retracted against base 104, valvestem 202 can extend slightly below a bottom surface 186 of supportstructure 114. Channels 212 may be formed in flexure ring 152surrounding aperture 208 and valve stem 202 to connect chamber 120 tovalve chamber 220. However, O-ring 206 is positioned around valve stem202 in valve chamber 220 between annular flange 204 and flexure ring152. In addition, spring 214 is positioned between annular flange 204and a ceiling 222 of valve chamber 220. Spring 214 biases the valve 200into a closed position. (as shown in FIG. 3A). More specifically, O-ring206 is compressed between annular flange 204 and flexure ring 152 toseal channels 212 from valve chamber 220, thereby isolating valvechamber 220 from lower chamber 120. However, if valve stem 202 is forcedupwardly (as shown in FIG. 3B), then O-ring 206 will no longer becompressed and fluid may leak through a gap 218 around the O-ring. Assuch, valve 200 will be open and valve chamber 220 and lower chamber 120will be in fluid communication via channels 212.

A CMP apparatus including carrier head 100 senses whether the substratehas been successfully vacuum-chucked to the carrier head as follows. Thesubstrate is positioned against the flexible membrane 118. Pump 52 binflates bladder 160 to a predetermined pressure, and then valve 58 b isclosed to isolate bladder 160 from pump 52 b. A first measurement of thepressure in bladder 160 is made by means of pressure gauge 56 b. Thenpump 52 c evacuates lower chamber 120 to create a low-pressure pocketbetween the flexible membrane and the substrate in order to vacuum chuckthe substrate to the carrier head. Then a second measurement of thepressure in bladder 160 is made by means of pressure gauge 56 b. Thefirst and second pressure measurements may be compared to determinewhether the substrate was successfully vacuum-chucked to the carrierhead.

Carrier head 100 is configured so that valve 200 will actuate if thesubstrate is present, and will not actuate if the substrate is absent.As shown in FIG. 3A, if the substrate is not present, then when chamber120 is evacuated, flexible membrane 118 will move upwardly and contactthe valve stem. However, since flexible membrane 118 is flexible and ispartly supported against support structure 114 when chamber 120 isevacuated, the flexible membrane will tend to wrap around the valvestem, and the upward tension force on valve stem 202 from flexiblemembrane 118 will be insufficient to overcome the downward spring forcefrom spring 204, and the valve 200 will remain closed. On the otherhand, as shown in FIG. 3B, if the substrate is vacuum-chucked to theflexible membrane, the relatively rigid substrate will press on valvestem 202. In this case, the upward tension force from flexible membrane118 and substrate 10 will overcome the downward spring force from spring204, and the valve 200 will open, thereby fluidly connecting lowerchamber 120 to valve chamber 220. This permits fluid to be drawn out ofbladder 160 through valve chamber 220 and lower chamber 120, andevacuated by pump 52 c.

It should be noted that spring 204 is selected to provide a downwardforce that is sufficient to counteract the upward force applied by themembrane alone, but insufficient to counteract the upward force appliedwhen a substrate is attached to the membrane. In general, the larger theaperture 210 in support structure 114, the stiffer the membrane 118, andthe farther the valve stem 202 extends past lower surface 176, the moreforce flexible membrane 118 will apply to the valve stem 202, and thelarger the spring constant of spring 204 will need to be. However, alower spring constant results in less stress on the substrate as thevalve is actuated.

Referring to FIG. 4, bladder 160 may initially be at a pressure P₁. Thefirst pressure measurement is made at time T₁ before pump 52 c begins toevacuate lower chamber 120. When chamber 120 is evacuated at time T₂,flexible membrane 118 is drawn upwardly. If the substrate is present,valve 200 remains closed, and the pressure in bladder 160 will remainconstant at pressure P₁, or even rise to a pressure P₂ if supportstructure 114 applies an upward force to compress the bladder 160. Thus,the pressure in bladder 160 measured by gauge 56 b will remain at orabove pressure P₁. On the other hand, if the substrate is present, thenvalve 200 is opened and fluid is evacuated from volume 160 so that thepressure measured by gauge 56 b falls to pressure P₃. Therefore, if thesecond measured pressure is less than the first measured pressure, thesubstrate is attached to the carrier head. However, if the secondmeasured pressure is equal to or larger than the first measuredpressure, the substrate is not attached to the carrier head.

Computer 60 may be programmed to store the two pressure measurements,compare the pressure measurements, and thereby determine whether thesubstrate was successfully vacuum-chucked to the carrier head. This canprovide an extremely reliable substrate detector that is not subject to“false” signals, e.g., indications that the substrate is absent when itis, in fact, present. In addition, the sensor is contained within thecarrier head behind the flexible membrane, so that the sensor does notprovide an opportunity for slurry to contaminate the interior of thecarrier head.

Referring to FIG. 5, in another implementation, carrier head 100 aincludes two or more valves 300, 310 connected in parallel between lowerchamber 120 and bladder 160. For example, the first valve can extendbetween lower chamber 120 and a first chamber 302, whereas the secondvalve can extend between lower chamber 120 and a second chamber 312. Apassage 320 in flexure ring 154 can connect first chamber 302 to secondchamber 312. Thus, chamber 120 will be connected to bladder 160 ifeither or both valves 300 is triggered. This implementation increasesthe sensitivity of the carrier head to the presence of the wafer, andprovides redundancy in case one valve becomes stuck. In addition, if thecarrier head includes three or more valves spaced at equal angularintervals around the carrier base, the substrate will not be tilted asit is lifted.

Referring to FIG. 6, in another implementation, carrier head 100 bincludes two or more valves 400, 410 connected in series between chamber120 and bladder 160. For example, the first valve can extend betweenlower chamber 120 and a first chamber 402, and a passage 420 throughflexure ring 152 can connect first chamber 402 to a passage 414 that issealed from a second chamber 412 by the O-rings of second valve 410. Thesecond chamber 412 is connected to bladder 160 by passage 156. In short,the input of first valve 400 is connected to chamber 120, the output ofthe first valve 400 is connected to the input of second valve 402 bypassage 420, and the output of second valve 410 is connected to bladder160. Thus, chamber 120 will be connected to bladder 160 only if bothvalves 400 and 410 are triggered. This implementation increases thesensitivity of the carrier head to the absence of the substrate and tosituations in which the substrate is not sufficiently firmly secured tothe flexible membrane, e.g., if the substrate is attached to theflexible membrane by surface tension alone, and not by vacuum-chucking,and tilts rather than actuating both sensors. The input passage 414 ofsecond valve 410 can be separated from chamber 120, while allowing thevalve stem of second valve 410 to extend into the chamber 120, byO-rings 416.

As shown in FIG. 7, a flexible diaphragm 430 can be used instead ofO-rings to separate passage 414 of second valve 410′ from the chamber120. Valve stem 202′ of valve 410′ can rest on diaphragm 430, and abumper 432 can be affixed to the underside of diaphragm 430. Flexiblediaphragm 430 is sufficiently elastic that when bumper 432 is pressedupwardly by flexible membrane 118, bumper 432 can be forced up intoaperture 210′ in support structure 114, thus forcing valve stem 202′upwardly to actuate second valve 410′.

Referring to FIG. 8, in another implementation, carrier head 100 cincludes two valves 500 and 510 connected in series between chambers 120and 108. This implementation would be appropriate for the carrier headdiscussed in pending U.S. Application Ser. No. 60/114,182, filed Dec.30, 1998. In this implementation, the valves 500 and 510 can be formedbetween flexure ring 152′ and an annular gimbal clamp 158, and multiplefluid passages through the gimbal rod and the flexure ring are notrequired. First valve 500 fluidly connects chamber 120 to a first valvechamber 502 via channels 508 in flexure ring 152′ surrounding valve stem506, second valve 510 fluidly connects chamber 108 to a second valvechamber 512 via channels 518 in gimbal clamp 158 surrounding valve stem516, and first valve chamber 502 is connected to second valve chamber512 by an unillustrated passage. The first valve 500 is biased open byspring 504, and second valve 510 is biased closed by spring 514. If thelower chamber 120 is evacuated and a substrate is vacuum-chucked to thecarrier head, then valve stem 506 of first valve 500 will be actuated topress O-ring 506 against gimbal clamp 158 to close the first valve, andthe pressure in chamber 108 will remain constant. On the other hand, ifthe lower chamber 120 is evacuated but no substrate is present, thenfirst valve 500 will remain open. If loading chamber 108 is alsoevacuated when chamber 120 is evacuated, e.g., in order to lift theentire substrate backing assembly and the retaining ring away from thepolishing pad, then valve stem 516 will be pressed against housing hub102. This generates a downward force on the valve stem which canovercome an upward force from spring 514 that presses O-ring againstgimbal clamp 158, causing the second valve 510 to open and thusconnecting loading chamber 108 to lower chamber 120. Fluid will thenflow out of loading chamber 108 via lower chamber 120. On the otherhand, if the loading chamber 108 is pressurized when chamber 120 isevacuated, e.g., to control the contact area and pressure on thesubstrate during polishing, then valve 510 will remain closed. In sum,the valve assembly will be actuated to connect loading chamber 108 tolower chamber 120 only if a substrate is not present and chamber 108 isevacuated. The drop in pressure in lower chamber 120 can be detected bypressure gauge 52 c to indicate that the substrate is not present.

Alternatively, carrier head 100 c could include a single valve thatopens when chamber 120 is evacuated if a substrate is present. In thiscase, the valve that separates chamber 108 from a pump or pressuresource can remain open so that chamber 120 does not entirely evacuate,thus preventing the membrane 118 from pull so far into chamber 120 thatthe substrate becomes overstressed and damaged.

Although in several implementations the valves are described asconnecting lower chamber 120 to bladder 160, the valve could be used toconnect any two chambers in the carrier head, or the valve can connect achamber in the carrier head to the ambient atmosphere. Moreover, thevalve can be biased opened or closed, so that the presence of thesubstrate can either close or open the valve, respectively, when thevalve is actuated. The valve can be positioned in parts of the carrierhead other than the flexure ring. For example, the valve can be offsetfrom the center of the carrier and attached to a base ring with thevalve chamber formed between the flexure ring and the base ring. Inaddition, the passages formed through the carrier head to provide thefluid connections are exemplary. For example, fluid communication can beprovided by a flexible hose that is coupled to fixtures on the housinghub and base ring, a first passage can connect the fixture on the basering to the valve chamber, and a second passage can connect the valvechamber to the bladder.

The present invention has been described in terms of a number ofpreferred embodiments. The invention, however, is not limited to theembodiments depicted and described. The scope of the invention isdefined by the appended claims.

1. A method for detecting the presence of a substrate in a carrier head,comprising: placing a substrate against a substrate receiving surface ofa flexible membrane in the carrier head, the flexible membrane defininga boundary of a first chamber; evacuating the first chamber, wherein ifa substrate is attached to the lower surface of the flexible membranewhen the first chamber is evacuated, a first movable structure thatforms part of a substrate detection system is actuated; and determiningwhether the substrate is chucked to the substrate receiving surfacedepending on whether the first movable structure was actuated.
 2. Themethod of claim 1, wherein determining whether the substrate is chuckedincludes measuring a first pressure of a volume in the carrier headbefore the evacuating step and measuring a second pressure of the volumein the carrier head after the evacuating step, and comparing the firstand second pressures.
 3. The method of claim 2, wherein actuating thefirst movable structure causes the second pressure to be a differentpressure than if the first movable structure were not actuated.
 4. Themethod of claim 2, wherein the first movable structure contacts an innersurface of the flexible membrane.
 5. The method of claim 2, wherein thefirst movable structure comprises a portion of a valve.
 6. The method ofclaim 5, wherein the valve regulates fluid flow through a passage in thecarrier head.
 7. The method of claim 6, wherein the passage fluidlycouples the first chamber to a second chamber.
 8. The method of claim 5,wherein the valve is biased in a closed position, and actuation of thevalve opens the valve.
 9. The method of claim 5, wherein the valve isbiased in an open position, and actuation of the valve closes the valve.10. The method of claim 5, wherein the valve is biased by a spring, andthe spring constant of the spring is selected so that a force from thespring is sufficient to counteract a force from the flexible membranewhen the substrate is not attached, but is insufficient to counteract aforce from the flexible membrane when the substrate is attached.
 11. Themethod of claim 5, wherein the portion of the valve extends through anaperture in a support structure.
 12. The method of claim 11, wherein theportion of the valve extends slightly beyond a lower surface of thesupport structure.
 13. The method of claim 11, wherein the supportstructure is movable relative to a base.
 14. The method of claim 5,wherein the portion of the valve contacts the upper surface of theflexible membrane if the first chamber is evacuated.
 15. The method ofclaim 5, wherein the flexible membrane wraps around a lower portion ofthe valve if the substrate is not present.
 16. A method for detectingthe presence of a substrate in a carrier head, comprising: placing asubstrate against a substrate receiving surface of a flexible membranein the carrier head, the flexible membrane defining a boundary of afirst chamber; evacuating the first chamber, wherein if a substrate isattached to the lower surface of the flexible membrane when the firstchamber is evacuated, at least one of a first movable structure and asecond movable structure that form part of a substrate detection systemis actuated; and determining whether the substrate is chucked to thesubstrate receiving surface depending on whether the first movablestructure was actuated.
 17. The method of claim 16, wherein thedetermining step indicates that the substrate is chucked if either thefirst movable structure or the second movable structure are actuated.18. The method of claim 16, wherein the determining step indicates thatthe substrate is chucked if both the first movable structure and thesecond movable structure are actuated.